This invention relates to the field of medical gas sampling, more particularly for protecting medical gas sampling devices from contamination, obstruction and interfering substances.
Some patients receiving automatic ventilation through an artificial air tube are monitored for respiratory problems by drawing a small sample flow of gas from the air tube and continuously analyzing the level of carbon dioxide (capnography) or other gases. Typically, a sampling tube is used to convey the sampled flow of gas from the air tube to the gas monitoring means, the diameter of the sampling tube being small compared to the diameter of the air tube.
Ventilated patients in the Intensive Care Unit (ICU) commonly have pulmonary problems with many secretions such as copious viscous sputum. These secretions are thixotropic gels which can and do solidify and adhere to a variety of surfaces, including the plastics used in medical tubing such as sampling tubes used in gas monitoring devices.
In order to draw gas from the air tube, a vacuum is applied to one end of the sampling tube using for example a pump in the gas monitoring device. This usually causes relatively high gas velocities to occur in the sampling tube which in turn frequently causes the above mentioned secretions to enter the small diameter gas sampling tube of capnographs and similar devices where they may partially or completely obstruct air flow. Secretions may also enter the small sensitive optical sensing chamber of the capnograph devices and interfere with measurements.
Clinicians claim that capnograph type devices are not useful in the ICU environment because the above mentioned problems make it necessary to clear the instrument or tubes as often as every 20 minutes. While surgery patients usually have no pulmonary problems, and they receive drugs which dry the secretions, nevertheless, their air way gases contain anesthetic agents which can also distort the gas analysis.